1. Field of the Invention
The present invention relates to optical recording mediums, and more specifically to a double-disk optical recording medium which maintains its position relative to the same laser source when data read/write operation is switched over between upper and lower recording layers.
2. Description of the Related Art
Japanese Patent Publication 2000-76681 discloses a servo tracking technique for recording high-density data. To achieve high density recording, data is recorded on land tracks as well as on groove tracks which alternate with the groove tracks. Japanese Patent Publication 2003-16648 discloses a high density recording technique using a multi-layered optical recording medium of the phase change type.
Also known in the art is a double-disk optical recording medium. As shown in FIG. 1, the known double-disk optical recording medium is constructed of a transparent lower disk D0 and an upper disk D1. A laser beam is directed to the lower disk from below, and on its opposite side, a plurality of groove tracks 3 of width WG0 and land tracks 4 of width WL0 are formed, each track being coated with a recording layer 4. Upper disk D1 comprises a base member 5, whose lower side is formed with a plurality of groove tracks 6 of width WG1 and land tracks 7 of width WL1, each of the tracks 6 and 7 being coated with a recording layer 8.
As shown in FIG. 2, the lower recording layer 4 is comprised of a first dielectric film 4A initially formed on the transparent base member 1. On the dielectric film 4A is an optical recording film 4B on which a second dielectric film 4C and a reflecting film 4D are successively formed. With the information recording side of the base member 5 facing upwards, the upper recording layer 8 is fabricated. Initially, a reflecting film 8D is sputtered on the base member 5. On the reflecting film 8D is successively formed a dielectric film 8C on which an optical recording film 8B and a dielectric film 8D are successively formed. The upper disk is then turned up-side down and cemented with the lower disk by means of an intermediate layer 9 so that the information-recording sides of the disks are facing each other and the groove tracks of each disk are respectively aligned with the land tracks of the other. In the prior art double-disk structure, the widths WG0, WL0, WG1 and WL1 are substantially equal to each other.
Data written on the lower disk is read by a laser beam which is focussed on the recording film 4B. This laser beam penetrates the recording film 4B and is partially reflected off the reflecting film 4D downwards to a photodiode, not shown. On the other hand, data written on the upper disk is read by using the same laser beam. In this case, the laser beam is directed through the lower disk to the upper disk. The reflecting film 4D of lower disk has such a thickness that partial energy of the incident beam passes through it and reaches the upper disk, where the beam is focused on the recording film 8B and reflected off the reflecting film 8D downwards through the lower disk to the photodiode.
If the light transmissivity of lower disk D0 is denoted as T0 and the light reflectivity of upper disk D1 is indicated as R1, the total reflectivity of the double-disk structure is given by T02×R1. In order to ensure sufficient optical energy for reading the upper disk, while ensuring sufficient energy for reading the lower disk, it is necessary that reflectivity R0 is at least 0.5. Therefore, the thickness of reflecting film 4D is in the range between 10 nm and 20 nm. Since no limitations are imposed on the reflectivity of the upper disk, the thickness of the reflecting film 8D is in the range between 100 nm and 200 nm.
However, the carrier-to-noise ratio of data recorded on lower disks differs significantly from the carrier-to-noise ratio of data recorded on upper disks. It is undesirable for recording/playback apparatus for using a double-disk optical recording medium having different operating characteristics between its recording disks.